This invention relates generally to respiration systems, and, more particularly, to respiration systems for the administration of intermittent positive-pressure breathing therapy, either as part of a life support system for critically ill patients, or on a regular basis for patients with certain chronic lung conditions. In a positive-pressure respirator system, the patient is supplied with air or other breathable gas mixture at a positive pressure with respect to atmospheric pressure, the positive pressure being removed to allow the patient to exhale after a desired volume has been delivered to the patient, or after a desired pressure is reached in the patient's lungs.
In situations in which the patient is incapable of initiating a breath, such systems can operate to supply the assistance of positive pressure during each inspiration phase of a timed breathing cycle. However, where the patient is capable of initiating inspiration, the application of positive pressure during inspiration is not timed, but is triggered by the patient's initial intake of a breath. Typically, in systems available heretofore, the supply of gas at positive pressure is initiated in response to the attainment of a predetermined low pressure. As is well known, the pressure at the point of delivery of the gas to the patient falls in an approximately exponential manner during the expiration phase of the breathing cycle, and when the patient takes a new breath, the pressure falls further and at an increased rate. Ideally the system should be responsive to this drop in pressure caused by the patient's intake of breath. Unfortunately, however, the absolute pressure at which this intake of breath occurs may vary from patient to patient, and even from breath to breath with the same patient. Consequently, in systems in which the positive-pressure inspiration phase is initiated in response to an absolute pressure measurement, the time of the start of the inspiration phase may not always correspond exactly with the patient's initial inspiratory effort. It will be appreciated that the patient can be caused considerable discomfort, and may lose much of the benefit of the therapy, by the application of positive pressure to his lungs either before expiration has been completed, or after he has tried to begin inhaling again.
The initiation of a positive-pressure inspiration phase based on an absolute pressure measurement is even more disadvantageous when certain variations of the basic positive-pressure system are employed. In one variation, known as "positive end expiratory pressure" (PEEP), the patient has to breathe out against a positive pressure with respect to the atmospheric pressure. This leaves the lungs at a positive pressure at the end of expiration, and inhibits lung collapse, which might occur in certain respiratory conditions. For other conditions and ailments, a variation known as "negative end expiratory pressure" (NEEP) may be employed. Here, the patient breathes out into a negative or vacuum pressure with respect to atmospheric pressure, and air is drawn from the lungs by the negative pressure. This variation might be used if the patient is unable to exhale readily because of an increased resistance to the flow of air from his lungs, or because of a weakened condition. It will be appreciated that, if the mode of operation of the respirator system is changed from the normal mode to either of the modes just described, then the absolute pressure at which the patient initiates the intake of a new breath will be different from mode to mode, and readjustment of the system will be necessary to ensure that the patient receives positive pressure at the proper time in the breathing cycle.
Accordingly, there is a need in the respiration art for a respiration system in which gas is delivered to the patient at positive pressure in response to a patient demand for a new breath, but completely independently of the absolute pressure measured at the point of delivery to the patient. The present invention fulfills this need.